CODEWITHSUNDEEP
f#
anon
anon

Reputation:

String.length and concatenated strings

The two expressions below produce the same output:

> ("hello" + " " + "world!");;
val it : string = "hello world!"

> "hello" + " " + "world!";;
val it : string = "hello world!"

Why then String.length works with the first but not with the second one?

> String.length ("hello" + " " + "world!");;
val it : int = 12

> String.length "hello" + " " + "world!";;

  String.length "hello" + " " + "world!";;
  ------------------------^^^

stdin(57,25): error FS0001: The type 'string' does not match the type 'int'

This was generated on FSI 14.0.22214.0

Upvotes: 0

Views: 3786

Answers (2)

anon
anon

Reputation:

This happens because functions bind more strongly than the (+) operator.

> String.length "hello" + " " + "world!"

Evaluates to:

> 5 + " " + "world!"

Which produces the same error:

> 5 + " " + "world!";;

  5 + " " + "world!";;
  ----^^^

stdin(1,5): error FS0001: The type 'string' does not match the type 'int'

Upvotes: 3

Richard
Richard

Reputation: 109100

The parentheses are overriding the normal operator precedence. In particular function & argument has a very high precedence, so in the latter case it is being evaluated as

(String.length "hello") + " " + "world!"

and then trying to add a number to a string.

Upvotes: 6

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